Ampeg B15NC bias issue?

[Leo_Gnardo]

One of the sockets had voltage climbing to over +30vDC before I stopped and starting looking through my stuff for fresh caps.

Holey sox! essentially zero bias, no wonder it goes red-plate. (Bias being the difference between grid & cathode voltage.)

I don't have any 0.022uf 400vDC values, just 0.01 and 0.047 so I'll order some and get back to this later in the week.

You could tack in either for a check, and make sure nothing else is amiss before you go ordering parts.

I'm thinking of buying some Mallory 150s and some Orange Drop 715s and flipping a coin just before it's time to solder them in. I think I'll also get a pair of JJ 6L6 for the musician to use as a known good pair for getting this amp running after years of storage and I'll encourage him to consider a pair of NOS tubes once we know the problems are sussed out. If you have any opinions to share about that I'm open to suggestions.

FWIW I've always been partial to the Mallory 150. I use 630V rated ones for everything, figure less chance of failure. I'm sure the amp will be perfectly fine with JJ 6L6's once you get the grid voltage under control. Might be a thought to increase the value of the cathode R's a tad, once you get the new tubes in place. Why? The old amps were designed for 110-117 VAC power and these days you commonly find 120 up to nearly 130 VAC at your outlets. That raises both the filament and hi voltage a bit, makes output tubes draw more current. You can counter this by raising your Rk's by 10 or 20%.

And what loudthud said, check those grid leak R's and replace 'em if they've drifted upwards.

Thank you for your help and advice.

Glad to be of help and I'm sure that goes for the rest of the team, right guys? I'm sure this B-15 will sound a treat once you have it sorted.

[mike_mccue]

Thanks you to both for the additional info

loud thud asked:
"Does the amp have the Dad's name engraved on the Ampeg logo that lights up?"

The amp has an old fashioned social "calling card", with the surname name printed in a fancy script, sandwiched in between the acrylic plates.

[loudthud]

Here's a pic of the B15 I currently own.

[mike_mccue]

I got a chance to work on the amp today.

I removed and replaced the 0.022uf coupling caps with fresh 0.022uF 630vDC caps, and I replaced the 270k grid leak resistors with fresh 270k 1 watt Metal Film resistors.

After that I tried using the old tubes again and the DC voltage on the grid was still a problem. I didn't let it climb very high this time but it was quick to raise in voltage.

I swapped the tubes for new pair of JJ 6L6GC and the voltages at pin 5 then hovered at 0.025vDc and 0.050vDC on the respective sockets, but one socket seemed to be inclined to slowly trend upwards.

I went ahead and replaced the cathode cap on the power tubes. The old one measured about 10uf at 1000kHz on my cap meter and I replaced it with a new sprague 50uF/50vDc as per spec and I also swapped out the cathode cap on the driver/splitter tube because the power tube cathode cap was so out of spec.

I have a Compu Bias meter so I used it to check the bias on the new tubes. The amp has been "on" for the past half hour and the bias seems to very stable with one tube drawing 58mA and the other 60mA with an estimated 22+/- watts of dissipation for the JJ 6L6GC tubes.


I'm thinking about swapping the power tube's cathode resistor so as to lower the dissipation a bit.




It occurs to me that I don't really understand what was happening with the old tubes; Was all that DC I saw with the old tubes, and is the little bit of DC I see on the new tubes, getting on to the grid from inside the tube? I am imagining the new coupling caps should effectively block the DC from the previous stage's plate.


Thanks again for all the help.

[Leo_Gnardo]

I'm thinking about swapping the power tube's cathode resistor so as to lower the dissipation a bit.

It occurs to me that I don't really understand what was happening with the old tubes; Was all that DC I saw with the old tubes, and is the little bit of DC I see on the new tubes, getting on to the grid from inside the tube? I am imagining the new coupling caps would effectively block the DC from the previous stage's plate.


Thanks again for all the help.

I'm sure you could run 'em a bit cooler.

Old tubes were dodgy. Age plus being run way past their specs due to leaky coupling cap - they're ready for the rubbish bin.

I wouldn't worry about minor DC offsets like what you are seeing. When it goes past say a volt I start to swap out parts.

It's been a pleasure. Enoy your B-15!

[mike_mccue]

Thanks Leo.

I played it for about an hour tonight and it seems like it is solid and stable.

[Jack Hester]

I was browsing the web for various bias supply circuits (I've already posted something about this in another thread), and came upon this one.

A while back, when I was repairing my B-12-N (cathode-biased amp), I had resolved all issues except one. The bias current was way high, and I could find no reason for it other than it was possibly coming from the wiper of my Hum Balance pot. So, I lifted the wiper from the cathode junction of the power tubes, and the bias fell to a normal current. Seems the wiper of my Hum Balance pot was grounded to the case of the pot. I removed it and replaced with a know good 100R pot (ungrounded wiper). That pot is still in the amp. The one that I removed would work just fine in a fixed bias amp, which I plan to add to my Oliver PA-100X amp, as soon as I can put my hands on it (the misplaced pot).

I didn't see where you had resolved your bias issue completely, but you may want to check your pot for the same problem, if you haven't. I'd check it anyway, even if your bias has settled.

Jack